What is maximum MAILQUEUE_BATCH_PERIOD?

Should be an easy answer to get.

isn’t it inside the config.php ?

For an ie. i have the following setup in my config file:

# define the amount of emails you want to send per period. If 0, batch processing
# is disabled and messages are sent out as fast as possible
define('MAILQUEUE_BATCH_SIZE', 0);

# define the length of one batch processing period, in seconds (3600 is an hour)
define('MAILQUEUE_BATCH_PERIOD', 1800);

Best regards,
Ruslán